The invention relates generally to optical systems used in laser printers. More particularly, the invention relates to adjusting the resonant characteristics of a torsion oscillator mirror.
Torsion oscillators with a mirrored surface are used in laser printers to scan a laser beam bi-directionally across a laser printer drum. Generally, the scan angle of the mirrored surface of the torsion oscillator varies sinusoidally with respect to time. The reflective element, or mirror, of the torsion oscillator is typically attached by elastic members to a rigid supporting mount. The elastic members suspend the mirror while allowing it to rotate over a limited range along a single axis. The elastic members act as a torsional spring, and together with the mass of the attached mirror, form a torsional spring-mass oscillator. A means of exciting oscillation is provided to the device, typically in the form of an electromagnetic drive current having alternating polarity.
The construction of a torsion oscillator determines its basic operating characteristics, such as resonant frequency (Fr), operating scan angle, resonant frequency drift with temperature, and the amount of power required to drive the oscillator to a desired scan angle while operating near or far from resonance. Since a torsion oscillator typically forms a highly efficient resonant system, it is necessary to drive the device with a driving frequency very near resonance. Otherwise, its scan amplitude becomes difficult to maintain at a constant level with feedback control and reasonable amounts of drive power.
Typical torsion oscillators have a relatively high Q, which results in a useful drive frequency range much narrower than the distribution of nominal resonant frequencies of “identical” units produced with reasonable manufacturing tolerances. For example, a typical adevice might have a useful drive frequency range of ±0.5% of the nominal resonant frequency. However, the expected distribution of frequencies from a sampling of units manufactured according to a single design might be ±10% of the nominal design resonant frequency. As a result, each system using a torsion oscillator must be adapted or custom tailored for that particular device's usable frequency range.
What is needed, therefore, is a torsion oscillator having a larger usable frequency range, so that the detrimental effects of device-to-device resonant frequency variations are minimized.